A thermal processing chamber as used herein refers to a device that heats objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and an energy source, such as a plurality of lamps, that emits thermal energy for heating the wafer. During heat treatment, the semiconductor wafers are heated under controlled conditions according to a preset temperature regime. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include temperature sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected band of wavelengths. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
In alternative embodiments, instead of or in addition to using radiation sensing devices, thermal processing chambers can also contain thermocouples for monitoring the temperature of the wafers. Thermocouples measure the temperature of objects by direct contact.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that various chemical and physical reactions can take place as the wafer is fabricated into a device. During rapid thermal processing, which is one type of processing, semiconductor wafers are typically heated by an array of lights to temperatures, for instance, from about 400° C. to about 1,200° C., for times which are typically less than a few minutes. During these processes, one main goal is to heat the wafers as uniformly as possible.
Problems have been experienced in the past, however, in being able to maintain a constant temperature throughout the wafer and in being able to control the rate at which the wafer is heated. If the wafer is heated nonuniformly, various unwanted stresses can develop in the wafer. Not being able to heat the wafers uniformly also limits the ability to uniformly deposit films on the wafers, to uniformly anneal films on the wafers, besides limiting the ability to perform various other chemical and physical processes on the wafers.
Some of the problems that have been experienced in the past relate to the fact that semiconductor wafers are frequently coated with films of materials that affect the reflectivity and absorptivity of the surface. For instance, there can be variations between different wafers, and there can also be variations within a single wafer as a result of patterns produced on the wafer during the semiconductor device fabrication sequence. When wafers are irradiated by electromagnetic radiation, these variations in optical properties result in variations in the ability of the wafer to absorb power and consequent variations in the temperature reached. This can degrade the repeatability of thermal processing and can also degrade the uniformity across any given wafer. For example, a wafer with different regions coated with different materials will have different power absorption characteristics in these regions.
As such, a need currently exists for an improved thermal processing chamber and process that is capable of more efficiently heating semiconductor wafers and that is capable of uniformly heating semiconductor wafers.